1. Field of the Invention
The present invention relates to a method and an apparatus for displaying an image on a plasma display panel (PDP), and more particularly, to a method and an apparatus for displaying an image on a PDP having reduced flicker and pseudo-contour.
2. Discussion of the Related Art
A PDP transforms an input electrical signal into image data by selectively illuminating discharge cells arranged in a matrix form.
A color PDP expresses gray levels by time dividing one field into a plurality of subfields.
A viewer may easily see flicker when the screen is large and the frequency of the input signal is small.
Hence, displaying an image from a phase alternate line (PAL) image signal, which has a 50 Hz frequency, on a PDP may generate a large amount of flicker. Accordingly, a large amount of flicker may be generated when the PDP is driven with a vertical frequency of 50 Hz using conventional subfield arrangements such as a minimum increase arrangement or a minimum decrease arrangement. Since the PDP's screen size cannot be adjusted, the flicker must be reduced by accounting for the low frequency.
Korean laid-open patent application No. 2000-16955 discloses a method for suppressing the generation of flicker in a PDP. As shown in FIG. 1, a subfield in one frame may be divided into two groups G1 and G2 that except for the least significant bit LSB subfield, may be constructed identically or similarly to each other. This method may reduce the flicker as compared to conventional subfield arrangement methods such as the minimum increase arrangement or the minimum decrease arrangement.
Referring to FIG. 1, the entire period of one frame is 20 ms, and the period for groups G1 and G2 is 10 ms. An idle period 1 exists at the end of group G1, and an idle period 2 exists at the end of group G2.
FIG. 2 shows an example where a part of the low gray level is realized with the subfield arrangement of FIG. 1.
As shown in FIG. 2, when low gray levels, such as the gray levels 0 to 11, are expressed with the subfield arrangement of FIG. 1, the time difference between the subfields corresponding to the least significant bit (LSB) and LSB+1 may be about several ms.
For example, for the gray level 3, the lowermost subfield SF1 in the first group G1 and the lowermost subfield SF1 in the second group G2 are turned on. In this case, the subfield SF1 in the first group G1 is the LSB subfield, the subfield SF1 in the second group G2 is the LSB+1 subfield, and the time difference between them is about 10 ms.
When expressing the low gray level with the subfield arrangement above and by applying error diffusion, there may be a large time difference of several ms between the LSB and LSB+1 subfields. This may generate significant pseudo-contour at the boundary between the gray levels of a moving image since the continuation time of illumination with such a time difference may be short.
FIG. 3 shows the mechanism in which the pseudo-contour is generated due to a moving image having neighboring gray levels of 4 and 3. As shown in FIG. 3, the pseudo-contour generates about 5 spots, and the difference between the maximum gray level 4 among the original gray levels and the distorted gray level is 2, 1, 3, 2, and 1.5, respectively, according to the occurring position, indicating the intensity of pseudo-contour generation. The gray level distorted as such while the image is moving results in the distortion of color, which a viewer may recognize as a color distortion having the shape of the contour.
Korean laid-open patent application No. 2003-39282 discloses technology to solve the problem of the above laid-open patent. As shown in FIG. 4, the subfields in one frame may be divided into three groups G1, G2, and G3, and the middle group G2 may have a smaller luminance weight than the lowermost subfields in the groups G1 and G3. This method may reduce the pseudo-contour as compared to the method of dividing the subfields into two groups, and it may reduce the flicker as compared to other subfield arrangement methods such as the conventional minimum increase arrangement or minimum decrease arrangement.
Referring to FIG. 4, the entire period of one frame is 20 ms, in which the first group G1 begins at 0 ms and ends before 8.5 ms, the second group G2 begins at 8.5 ms and ends before 10.8 ms, and the third group G3 begins at 10.8 ms and ends before 20 ms. Three idle periods are located at the ends of the three groups G1, G2, and G3.
FIG. 5 shows an example in which a part of the low gray levels is realized with the subfield arrangement of FIG. 4.
As shown in FIG. 5, when low gray levels, such as the gray levels 0 through 11, are expressed with the subfield arrangement of FIG. 4, the LSB and LSB+1 subfields are located in the middle subfield group G2 to reduce the time difference between the subfields, which reduces the pseudo-contour at the boundary between the gray levels while the image at the low gray level is moving.
The PDP consumes a large amount of power due to its driving characteristic, so Automatic Power Control (APC) technology may be used to control the power consumption according to the load ratio or the average signal level of a frame to be displayed. The APC technology is a method in which the APC levels change according to the load ratio of the input image data, and the number of sustain pulses change at each APC level to restrict the power consumption to below a certain level.
According to the APC technology, therefore, the number of sustain pulses applied to the respective subfields change according to the load ratio. Referring to the subfield arrangement of FIG. 4, all sustain pulses applied to the groups G1, G2 and G3 are changed according to the load ratio. Accordingly, the subfields have a number of sustain pulses corresponding to their luminance weight, so the number of sustain pulses applied to the respective subfields may also change.
FIG. 6 shows the position of subfield groups and the central position of illumination at each load according to the conventional subfield construction of FIG. 4, in which (a) shows the case of the minimum load ratio, and (b) shows the case of the maximum load ratio.
As shown in (a) of FIG. 6, in the case of the minimum load ratio, the time difference A between the central positions of illumination from the first group G1 to the third group G3 is larger than the time difference B between the central positions of illumination from the third group G3 to the first group G1. As shown in (b) of FIG. 6, in the case of the maximum load ratio, the time difference C between the central positions of illumination from the first group G1 to the third group G3 is larger than the time difference D between the central positions of illumination from the third group G3 to the first group G1. Consequently, the time difference between the central positions of illumination of the groups G1 and G3, which are the more important subfield groups in the frame, is larger than the time difference between the two groups in successive frames irrespective of the load ratio.
Accordingly, there is a problem in the conventional art where when dividing one frame into three subfield groups, flicker may easily occur since the fixed starting positions St_2 and St_3 of the second and third subfield groups may cause a lack of periodicity of the central position of illumination.